The summer travel season, with millions of Americans jamming airline flights, has been marked by record numbers of delays. Planeloads of people have been forced to sit on runways for hours, others have been stranded when flights were canceled.

The airlines blame the air traffic control system. The Federal Aviation Administration blames bad weather and equipment problems. Air traffic controllers blame the airlines for scheduling too many flights at the same time.

"When it comes to getting to the bottom of delays, the air is thicker with accusations than with aircraft," Sen. Richard C. Shelby, Alabama Republican, said yesterday as his Appropriations Committee transportation panel took on the issue. "Passengers just know the simple truth that air travel is costly, unpleasant and less reliable than they would like it."

FAA figures show 48,448 delayed flights in June, a month of repeated heavy thunderstorms in the Midwest. That was up from 41,602 delays in June 1999.

But that's not the half of it, according to Transportation Department Inspector General Kenneth M. Mead.

The statistics aren't reliable, he said and delays are far higher than reported.

If an airplane leaves its gate within 15 minutes of the scheduled time, that counts as leaving on time "even if it then sits on the runway for three hours," Mr. Mead said.

And those long waits on the runway aren't always the fault of weather or air traffic controller delays, he said.

The airlines schedule more planes for popular takeoff times than can be accommodated, knowing there will be a wait, he complained. Then, to get listed as on time, they pull away from the terminal, get in line and sit.

In addition, Mr. Mead said airlines have changed their anticipated flight times for example, listing one that used to take 90 minutes as now taking two hours to reduce the number of late arrivals.

If the airlines were using the same scheduled flight times they used in 1988, late arrivals would be 25 percent higher than they are now, he said.

The committee's witness list originally included Carol Hallett, president of the Air Transport Association, which represents the nation's major airlines. However, the ATA said Miss Hallett was out of town and could not attend.

ATA spokesman Dave Fuscus, in a later telephone interview, put forward the industry view that the delays are largely the fault of an antiquated air traffic control system.

He said the airlines lengthened their scheduled air times to reflect the reality of a system that cannot move traffic faster. As to leaving the gate when they know there is a wait to take off, he said: "If there is a line to take off, you want to get in line as soon as you can."

FAA Administrator Jane Garvey said that while bad weather is the most common cause of delays, other reasons include inoperable runways, airport capacity limits, aircraft maintenance and crew problems, as well as air traffic equipment problems and procedures.

"Delays will never be eliminated," she said, but the goal is to minimize them.

Edward Kragh, an air traffic controller from Newark, N.J., said, "Airlines have embarked on a well-financed campaign of misinformation blaming air traffic control for their delays.

"As long as the airlines continue to overbook runways, especially during peak hours, air traffic delays will continue and passengers will wait," he said.

Mr. Shelby condemned the "hurry up and wait" mentality that pervades air travel:

"We're told to get to the airport at least an hour before flight time only to wait in line to be processed.

"We're told to be at the gate at least 20 minutes before flight time or our seat will be forfeited only to wait in line to be herded on the plane like domesticated animals.

"We're told to be belted in our seats five minutes before the scheduled departure only to pull away from the gate and be held hostage until the airlines' real schedule or the system can accommodate us."